(1) Field of the Invention
The present invention relates generally to a semiconductor device and, more particularly, to a semiconductor device used in a high frequency amplifier.
(2) Description of the Prior Art
In recent years, high frequency active semiconductor elements, such as bipolar transistors or GaAs FETs (which is an abbreviation of gallium arsenide field effect transistors), are increasingly employed in a high frequency amplifier, such as a microwave amplifier. Generally, a GaAs FET amplifier requires two direct bias sources, which results in a complex construction of the amplifier. In order to avoid such a complex construction, a self-biased circuitry configuration in which only one direct bias source is necessary and the other bias voltage is automatically provided is widely used. For example, in the case of an amplifier formed by a bipolar transistor, such as an npn-type transistor, a collector and an emitter thereof are connected to ground and a negative bias source, respectively, and a negative base bias voltage is provided automatically by the flow of current through a bias circuit arranged between the collector and the emitter. In this case, a bypass capacitor for providing a low-impedance path for high frequency current is connected between the emitter and ground, so that the amplification characteristics are stable and efficient. On the other hand, in the case of an amplifier formed by a GaAs FET, a drain thereof is connected to a positive bias source, while a source and a gate thereof are connected to the ground through a resistance. As a current flows from the drain to the source and finally to the ground via the resistance, the potential of the source is positive. On the other hand, the potential of the gate is equal to the ground potential. Therefore, a negative voltage is automatically provided between the gate and the source. In this case, a bypass capacitor is also connected between the source and ground, so that the amplification characteristics are stable and efficient.
One conventional amplifier, which is formed on a printed circuit board comprises an active semiconductor element, such as a bipolar transistor or a GaAs FET, a bypass capacitor which is connected to one electrode of the semiconductor element through a metalized thin conductive layer, a terminal plate and other circuits such as matching circuits. However, in such an amplifier, since a path between the active semiconductor element and the capacitor is relatively long, a stray inductance or a parasitic inductance is generated in the path at a high frequency. In this case, the higher is the frequency of the current flowing through the path, the greater is the value of the parasitic inductance thereof. Therefore, in a relatively high frequency range, the impedance between the terminal of the semiconductor element and ground becomes so large that high frequency currents can not sufficiently pass through the bypass capacitor, which results in the amplification characteristics being unstable and degraded. In addition, if the active semiconductor element is of a high power, the element must be provided with a heat sink for dissipating heat, which sink is of a large scale. As a result, it is difficult to arrange a bypass capacitor in the proximity of a high power semiconductor element due to the presence of the large heat sink, which also results in the amplification characteristics being unstable and degraded.